This proposal is focused upon the intracellular amastigote stage of Leishmania parasites. Disease pathology associated with infection is due to mechanisms of and response by the host to the amastigote stage of the parasite. Research during the past period of support has demonstrated that immunization with molecules specifically expressed or upregulated in the amastigote stage can confer resistance to infection. Interestingly, recent data indicate that amastigotes residing within the macrophage parasitophorous vacuole can sequester their antigens from MHC class II presentation (CD4+ T cells). However, CD8+ (as well as CD4+) T cells are critical resistance/protection against infection in immunized/vaccinated C57BL/6 mice. Consequently, the hypothesis to be examined in this proposal is that: CD8 T cells are important in vaccinated (immune) mice for protection against infection. This hypothesis will be examined using mice of different genetic background (susceptibility to infection) and both protective and non-protective antigens. Further, the CD8 T cell mechanisms potentially contributing to protection and their effect on CD4 T cell activation will be explored. The contributions of CTL activity and of TC1 versus TC2 CD8+ T cells will be examined. The ability of CD8+ T cells to amplify and/or modulate the CD4+ response [TCR usage; cytokine profile] will be determined. In addition, the biological roles of the protective amastigote antigens [P-4 and P-8] will be investigated. These studies will focus on the Leishmania mexicana complex.